Definition of the target sign and its use for the diagnosis of schwannomas.

UNLABELLED: The target sign is one of the characteristic imaging findings in schwannomas that distinguishes it from other soft tissue tumors, but its definition has varied in past studies. We defined the characteristic target sign on MRI and clarified its sensitivity and specificity based on histology. Of the 199 schwannomas reviewed, 162 (81%) showed biphasic macroscopic and microscopic patterns of central Antoni A and peripheral Antoni B cells; 118 (59%) also showed a biphasic pattern on MRI. Gadolinium-enhanced T1-weighted images showed central high intensity and peripheral low intensity, whereas T2-weighted images showed peripheral high intensity and central low intensity. Seventy-eight cases had cystic, hemorrhagic, or necrotic degeneration, which corresponded to high intensity on T2-weighted images and low intensity on gadolinium-enhanced T1-weighted images. Nine cases in which the degenerative area was only in the central portion of Antoni A showed a triphasic pattern. We defined the target sign as the biphasic or triphasic pattern on MRI, and the sign correlated with macroscopic and microscopic findings. The specificity of the target sign in schwannoma was 100% and the sensitivity was 59%; therefore, the target sign was characteristic of and helpful for the diagnosis of schwannomas. LEVEL OF EVIDENCE: Level II, diagnostic study. See the Guidelines for Authors for a complete description of levels of evidence.